Abstract

The propagation of light in a multimode optical fiber waveguide results in speckle, a complicated mapping of the input field onto the output field. The speckle pattern changes as the fiber undergoes twists and bends. We show theoretically and demonstrate experimentally that changes in the mapping caused by bends in one region of the fiber can be compensated with bends applied along a fixed region of the fiber. In this way the output speckle pattern of the fiber can be stabilized. In this method it is supposed that the modes evolve adiabatically without intermixing and that the analytical dependence of perturbed propagation eigenvalues of the modes follows R−2 behavior, where R is the local value of bend radius.

Figures (4)

Schematic of compensation of the bend-induced distortions: a, strongly prebent auxiliary and the straight utility parts, which define the transmission mapping to be stabilized; b, influence of bending of the utility part, which may be compensated by the actuator-controlled (A) loosening of the auxiliary part of the fiber.

(a) Starting output speckle pattern in the far-field zone; (b) output pattern, strongly distorted by bending of the utility portion of the fiber (the left-hand part of Fig. 2 shifted into the dashed position); (c) loosening of the bend of the auxiliary portion (the right-hand part of the fiber on Fig. 2 is also shifted into the dashed position), permitting compensation for distortions.

Dependence of the intensity of the reconstructed plane wave on the position of the utility part of the fiber during distortion (squares) and on the position of the auxiliary part during the compensation process (circles). The maximum of the second curve corresponds to the best compensation.